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zope.session 4.1.0

zope.session

This package provides interfaces for client identification and session
support and their implementations for zope.publisher’s request objects.

Sessions

Sessions provide a way to temporarily associate information with a
client without requiring the authentication of a principal. We
associate an identifier with a particular client. Whenever we get a
request from that client, we compute the identifier and use the
identifier to look up associated information, which is stored on the
server.

A major disadvantage of sessions is that they require management of
information on the server. This can have major implications for
scalability. It is possible for a framework to make use of session
data very easy for the developer. This is great if scalability is not
an issue, otherwise, it is a booby trap.

Design Issues

Sessions introduce a number of issues to be considered:

Clients have to be identified. A number of approaches are possible,
including:

o Using HTTP cookies. The application assigns a client identifier,

which is stored in a cookie. This technique is the most
straightforward, but can be defeated if the client does not
support HTTP cookies (usually because the feature has been
disabled).

o Using URLs. The application assigns a client identifier, which is

stored in the URL. This makes URLs a bit uglier and requires some
care. If people copy URLs and send them to others, then you could
end up with multiple clients with the same session
identifier. There are a number of ways to reduce the risk of
accidental reuse of session identifiers:

Embed the client IP address in the identifier

Expire the identifier

o Use hidden form variables. This complicates applications. It

requires all requests to be POST requests and requires the
maintenance of the hidden variables.

o Use the client IP address

This doesn’t work very well, because an IP address may be shared by
many clients.

Data storage

Data can be simply stored in the object database. This provides lots
of flexibility. You can store pretty much anything you want as long
as it is persistent. You get the full benefit of the object database,
such as transactions, transparency, clustering, and so on. Using
the object database is especially useful when:

Writes are infrequent

Data are complex

If writes are frequent, then the object database introduces
scalability problems. Really, any transactional database is likely
to introduce problems with frequent writes. If you are tempted to
update session data on every request, think very hard about it. You
are creating a scalability problem.

If you know that scalability is not (and never will be) an issue,
you can just use the object database.

If you have client data that needs to be updated often (as in every
request), consider storing the data on the client. (Like all data
received from a client, it may be tainted and, in most instances,
should not be trusted. Sensitive information that the user should
not see should likewise not be stored on the client, unless
encrypted with a key the client has no access to.) If you can’t
store it on the client, then consider some other storage mechanism,
like a fast database, possibly without transaction support.

You may be tempted to store session data in memory for speed. This
doesn’t turn out to work very well. If you need scalability, then
you need to be able to use an application-server cluster and storage
of session data in memory defeats that. You can use
“server-affinity” to assure that requests from a client always go
back to the same server, but not all load balancers support server
affinity, and, for those that do, enabling server affinity tends to
defeat load balancing.

Session expiration

You may wish to ensure that sessions terminate after some period of
time. This may be for security reasons, or to avoid accidental
sharing of a session among multiple clients. The policy might be
expressed in terms of total session time, or maximum inactive time,
or some combination.

There are a number of ways to approach this. You can expire client
ids. You can expire session data.

Data expiration

Because HTTP is a stateless protocol, you can’t tell whether a user
is thinking about a task or has simply stopped working on it. Some
means is needed to free server session storage that is no-longer needed.

The simplest strategy is to never remove data. This strategy has
some obvious disadvantages. Other strategies can be viewed as
optimizations of the basic strategy. It is important to realize that
a data expiration strategy can be informed by, but need not be
constrained by a session-expiration strategy.

Zope3 Session Implementation

Overview

Caution!

Session data is maintained on the server. This gives a security
advantage in that we can assume that a client has not tampered with
the data. However, this can have major implications for scalability
as modifying session data too frequently can put a significant load
on servers and in extreme situations render your site unusable.
Developers should keep this in mind when writing code or risk
problems when their application is run in a production environment.

Sessions allow us to fake state over a stateless protocol - HTTP.
We do this by having a unique identifier stored across multiple
HTTP requests, be it a cookie or some id mangled into the URL.

The IClientIdManager Utility provides this unique id. It is
responsible for propagating this id so that future requests from
the client get the same id (eg. by setting an HTTP cookie). This
utility is used when we adapt the request to the unique client id:

Data Storage

The actual data is stored in an ISessionDataContainer utility.
ISession chooses which ISessionDataContainer should be used by
looking up as a named utility using the package id. This allows
the site administrator to configure where the session data is actually
stored by adding a registration for desired ISessionDataContainer
with the correct name.

If no ISessionDataContainer utility can be located by name using the
package id, then the unnamed ISessionDataContainer utility is used as
a fallback. An unnamed ISessionDataContainer is automatically created
for you, which may replaced with a different implementation if desired.

The ISessionDataContainer contains ISessionData objects, and
ISessionData objects in turn contain ISessionPkgData objects. You
should never need to know this unless you are writing administrative
views for the session machinery.

Session Timeout

We need to keep up with when the session was last used (to know when it needs
to be expired), but it would be too resource-intensive to write the last access
time every, single time the session data is touched. The session machinery
compromises by only recording the last access time periodically. That period
is called the “resolution”. That also means that if the last-access-time +
the-resolution < now, then the session is considered to have timed out.

The default resolution is 10 minutes (600 seconds), meaning that a users
session will actually time out sometime between 50 and 60 minutes.

Use zope.schema.fieldproperty.FieldProperty for “namespace” attribute of
CookieClientIdManager, just like for other attributes in its interface.
Also, make ICookieClientIdManager’s “namespace” field an ASCIILine, so
it accepts only non-unicode strings for cookie names.

3.9.1 (2009-04-20)

Restore compatibility with Python 2.4.

3.9.0 (2009-03-19)

Don’t raise deprecation warnings on Python 2.6.

Drop dependency on zope.annotation. Instead, we make classes implement
IAttributeAnnotatable in ZCML configuration, only if zope.annotation
is available. If your code relies on annotatable CookieClientIdManager
and PersistentSessionDataContainer and you don’t include the zcml classes
configuration of this package, you’ll need to use classImplements function
from zope.interface to make those classes implement IAttributeAnnotatable
again.

Drop dependency on zope.app.http, use standard date formatting function
from the email.utils module.

Zope 3 application bootstrapping code for session utilities was moved into
zope.app.appsetup package, thus drop dependency on zope.app.appsetup in this
package.

3.8.1 (2009-02-23)

Add an ability to set cookie effective domain for CookieClientIdManager.
This is useful for simple cases when you have your application set up on
one domain and you want your identification cookie be active for subdomains.

3.8.0 (2008-12-31)

3.7.1 (2008-12-30)

ZODB 3.9 no longer contains
ZODB.utils.ConflictResolvingMappingStorage, fixed tests, so they
work both with ZODB 3.8 and 3.9.

3.7.0 (2008-10-03)

New features:

Added a ‘postOnly’ option on CookieClientIdManagers to only allow setting
the client id cookie on POST requests. This is to further reduce risk from
broken caches handing the same client id out to multiple users. (Of
course, it doesn’t help if caches are broken enough to cache POSTs.)

3.6.0 (2008-08-12)

New features:

Added a ‘secure’ option on CookieClientIdManagers to cause the secure
set-cookie option to be used, which tells the browser not to send the
cookie over http.

This provides enhanced security for ssl-only applications.

Only set the client-id cookie if it isn’t already set and try to
prevent the header from being cached. This is to minimize risk from
broken caches handing the same client id out to multiple users.